Nowadays temperature sensitive controls have been widely applied in a variety of apparatus or appliances, such as electrical cooking devices, electrical power switches, thermostatic devices. The temperature sensitive control may be classified in accordance with the temperature sensitive material thereof into three categories: (1) bi-metal temperature sensitive control, (2) shaped SMA temperature sensitive control and (3) metal powder driving type temperature sensitive control. These temperature sensitive controlling devices may also be classified as linear type and non-linear type in accordance with the characteristic curve thereof. The term "characteristic curve" used herein is referred to the relationship between the elongation or displacement of the controlling device and the temperature variation thereof.
The bi-metal type temperature sensitive controlling device usually has a linear temperature characteristic curve as shown in FIG. 1. Such a controlling device is generally used in electrical switches or protective circuits and the action thereof in response to temperature variation is an ON-OFF type.
The characteristic curve of a shape memory alloy is generally non-linear, such as the one shown in FIG. 2. The general usage of the kind of temperature sensitive controlling device is to provide a switch between an ON and an OFF states. The conventional SMA spring or strip is made of a copper-based alloy. The transition points, which are the temperature at which the SMA changes its shape due to the change of the metallurgical composition thereof and subsequent heat treatment, of the copper-based SMA varies with the frequency of operation and thus making the overall control unstable. Thus such an SMA type temperature sensitive controlling device cannot be adapted on an apparatus which requires frequent switching ON and OFF. This kind of temperature sensitive controlling device is usually used in apparatus with less-frequent switching, such as fire or temperature warning devices or water temperature sensing devices.
The metal powder driving type temperature sensitive controlling device uses high thermal conductivity copper powders to provide a linear response to temperature variation. The slope of the characteristic curve, however, is quite small and thus the response to temperature variation is slow. Currently, such a temperature sensitive controlling device is generally adapted on water temperature controls.
The present invention provides a temperature sensitive controlling device which is made of powders of at least two shape memory alloys of different transition points. By mixing appropriate amounts of these powders together, a characteristic curve different from those of the base shape memory alloys can be obtained. By varying the ratio between theses powders, the characteristic curve can be modified to a desired shape. Thus, the present invention can be used to substitute all the prior art temperature sensitive controlling devices. Therefore, the present invention provides a temperature sensitive controlling device superior to the prior art devices.